Isolated explants from the animal hemisphere of Xenopus embryos were incubated with Xenopus basic fibroblast growth factor (XbFGF) or human activin A. XbFGF incubation resulted in the rapid activation of mitogen-activated protein kinase (MAPK) and ribosomal S6 protein kinase (pp90rsk) in a dose-dependent manner with the highest levels of activation occurring at 50 ng/ml. Maximal activation occurred within 6-10 min after the addition of growth factor, and the activity of both kinases declined to unstimulated levels after 30 min. Activin was unable to activate either MAPK or pp90rsk in the Xenopus explants to a substantial level, although it induced dorsal mesoderm better than XbFGF under the same experimental conditions. The regulatory protein Xwnt-8 did not activate MAPK, nor did it enhance the activation of MAPK by XbFGF. XbFGF was able to activate MAPK through at least the midgastrula stage, suggesting that this family of growth factors may have a role in gastrula-stage events.